1. Field of the Invention
The present invention relates to high frequency DC to AC switchmode power converters and specifically to high frequency ballasts for gas discharge devices. More specifically, the present invention relates to a high frequency ballast for high pressure sodium lamps.
2. Prior Art
Self-oscillating DC-to-AC converters have a significant position in the field of switchmode power converters, due to their simplicity and usefulness. Generally, DC-to-AC converters are configured as push-pull, half-bridge or full-bridge. One of the simplest, and oldest, DC-to-AC self-oscillating push-pull converters is the Royer circuit. Another topology similar to the Royer circuit, which removes the switch drive function from the main power transformer, is the self-oscillating voltage or current driven Jensen circuit. The common disadvantage of the push-pull configurations is the imbalance problem of the push-pull transformer, especially when applied to asymmetrical loads.
An important application of the simple self-oscillating DC-to-AC switchmode power converters is supplying gas discharge devices, especially high pressure sodium (HPS) lamps in the range of 35 to 400 watts. In this case, the load impedance of the DC-to-AC converter is a HPS lamp connected in series with an inductor. In the case of a high frequency powering of the HPS lamp, the interaction between the high frequency ballast and the lamp is stronger than that of a conventional ballast. This high frequency ballast is significantly better than a conventional ballast due to its lessened weight and higher efficiency. Additionally, the high frequency ballast, utilized with an HPS lamp would have a longer life time, exhibit better light efficiency (lumen per watt) and display a better color temperature.
Therefore, the critical design targets for high frequency ballasts supplying HPS lamps would be the following:
(a) very high efficiency (energy saving);
(b) ensuring that the lamp power is maintained between an allowed maximum and minimum power during the lifetime of the lamp at .+-.20% input voltage fluctuation;
(c) protection against the imbalance effect caused by the asymmetrical loading feature of the ignited HPS lamp;
(d) providing high voltage (3000V-4000V) ignition pulses;
(e) the relative simplicity of the ballast which would result in a lower cost;
(f) reliability and longer life time; and
(g) eliminating the possibility of acoustic resonance by using frequency modulation.
The prior art is replete with many known push-pull configurations providing high frequency ballast for gas discharge lamps. A typical Jensen push-pull which can be used with HPS lamps is U.S. Pat. No. 4,935,673 entitled "Variable impedance electronic ballast for gas discharge device", assigned to the assignee of the present invention, including an improved current driven Jensen push-pull converter.